Just like Windows, prolonged use of a Mac slows down the operating system. When you’ve been hoarding data and applications for over a year, the difference in performance starts to show. It just doesn’t run as smoothly as it once did.

A lot of users are hesitant to reinstall their entire operating system. There are plenty of advantages to a fresh install, especially performance-wise, but it’s a much bigger undertaking to start over from scratch.

Despite the work involved, it’s definitely worth it in the long run and here we will run through the process from start to finish. It’s a three-step undertaking: back-up, install and reconfigure.

(Optional) Start With Cleaning

It’s perhaps best to add in a hidden fourth step before we get started. Using your computer for a long time, you likely have a lot of stuff you don’t need on your hard drive. Applications you have installed but didn’t keep using and files that became redundant months ago. Take this time to sort through your data and throw away everything you don’t want to keep.

If your files are disorganised (that is, you have amassed a few giant folders that lack organisation) take this time to sort them out. Separate work documents from game installers and family photos. Classify and organise your files in a few typed folders. Of course, this is entirely optional, but it’ll help in speeding up the back-up process and will get you up and running that much faster on your next install.

1. Backing Up Your Data

There’s a difference between starting with a clean slate and starting entirely from scratch. Naturally, there’s data you’d like to migrate from your old OS X installation. Before you reinstall your computer, make sure your data is safe.

This is a good option if you only want to move a set number of folders between your two installations, or if the disk isn’t large enough to house all of your data and it needs to be split up into several parts. Otherwise, the better option is to use Time Machine (see below).

Also take into account application data. It’s tempting to just back up the files on your desktop and in your Documents folder and to forget about the data kept by iTunes and iPhoto. You can find these application libraries in the Music and Pictures folders located in your home folder, respectively.

1.b Time Machine

Time Machine is Apple’s official back-up solution. Although people often use Apple’s own Time Capsule hardware, you can use your own external hard drive with Time Machine as well. If your disk is large enough, Time Machine provides a complete all-in-one back-up solution. Unless specified otherwise, Time Machine will make a back-up of all your files. You can choose which folders to put back after reinstalling your system.

You can configure Time Machine by heading to the Time Machine tab in the System Preferences. Make sure Time Machine is enabled and select the disk of your preference. It may take as much as an hour for the first back-up to start (the time of the next back-up is shown at the top of the preferences pane). Also check the options to make sure no important folders are left out.

2. Installing Mac OS X

You may have received an install DVD with your Mac when you bought it. You can use that DVD to install Mac OS X and upgrade it to the latest version using the Mac App Store. Alternatively, you can install the latest version straight away.

We’ll take you through the basic steps. You can find a more detailed walkthrough in MakeUseOf’s Mac OS X Lion guide.

2.a Boot the Mac OS X Recovery Partition

If you’re running Mac OS X 10.7 Lion or newer, we can reinstall Mac OS X without an installation disk. That’s because new Macs come with a recovery partition that houses a variety of tools, including access to Disk Utility and an online Mac OS X installer.

To run the Mac OS X recovery partition, restart your system and hold the Cmd+R keys while the computer reboots.

Note: If you’re running a Mac OS X version lower than 10.7, or if you don’t have access to the internet during the installation process, you should follow the alternative steps below to create a USB installer thumb drive.

OR (Alternative) Create a USB Mountain Lion Installer

To create a USB installer, you’ll need access to the Mac App Store and an empty USB thumb drive of at least 8GB.

If you’ve purchased and installed Mac OS X Mountain Lion in the past, the installer won’t be on your computer anymore. By going to the Mac App Store and searching for Mountain Lion, you can simply download the installer once more. When the download is finished, don’t start the installation just yet.

Download Lion DiskMaker (free) and run it. It will ask you which version of Mac OS X you’d like to install (likely Mountain Lion) and it will automatically locate the installer for you. Follow the instructions of the application and when you’re done, you’ll have the installer on your thumb drive.

Reboot your computer and hold the option (alt) key. You’ll see the different available boot options. Choose the installation disk and wait for it to load.

2.b Erase Your Disk

Before you do this, make sure you’ve backed up your data as outlined in step 1. This step will erase all data on your main disk!

After the welcome screen and language select, you’ll be presented a few options. Select Disk Utility. From the left sidebar select the current main partition. Toward the top of the Disk Utility window, select Erase.

Closing Disk Utility will take us back to the options selection.

2.c Install Mac OS X

Now you can select the Reinstall Mac OS X option. Select the disk or partition you just formatted as the target location and follow the instruction provided by the installer. Installing an operating system always takes a while, but you’ll be glad to hear that the process is significantly sped up by using the recovery partition or a thumb drive instead of an old-fashioned disc!

3. Reconfiguring Mac OS X

When Mac OS X Mountain Lion has finished installing, it should automatically launch the Migration Assistant. Don’t worry if it didn’t or if you closed it already, you can always find it in the Applications > Utilities folder.

The Migration Assistant will help you put back the data that you backed up in step one. Select the source of your back-ups (this will be the Time Machine instance, or other disk) and the data you’d like to put back. The Migration Assistant will even help you put back in place the user accounts of your previous installation.

If you didn’t back up your applications, open the Mac App Store and head to the Purchases tab. Here you’ll be able to reinstall all your previous purchases in a jiffy.

Once you’re done restoring files and applications, it’s time to start playing around with your fresh install. Maybe you now have room left for other applications. Be sure to check out our top Mac apps list. But don’t go overboard, or you’ll soon be aching for a fresh install again.

How often do you reinstall the operating system on a computer and what is the main reason? Let us know in the comments below.

That's where everything gets much more complex and research - time required to know how each key or security function works with the OS. Just reinstalling some certain vendors apps can be a pain or downright confusing even for the most experienced.

I've done it were I have used Carbon Copy Cloner (way better than time machine)
and then manually done the work - migration assistance was a fail - it's been a fail even as far back as OS9 x. Never worked the way advertised. But I can't speak for the current OS I use - 10.6.8 .Have not done a clean install on this machine yet.

A major advantage of a tower computer is you can keep adding hard drives. I've been doing a clean install on a shiny new hard drive since about System 10.2. Eventually, you can toss out everything but the documents on the old drives and reuse them. But, heck, a 1 TB hard drive costs about $80. Maybe I'll try a SSD next time.

Personally I thought it was much easier to choose the files I wanted to save and put them in a folder. From there I created a new user, transferred the files over by placing them in the shared spot. Logged into the new account, transferred files over then deleted the old user account. Computer is at least twice as fast if not more

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